This invention relates to the manufacture of anhydrous alkali dithionites by reacting an alkaline formate, an alkali metal agent, and sulfur dioxide in an alcohol/water solvent. It particularly relates to improving this process by reusing aqueous column bottoms from the solvent recovery distillation to produce additional dithionite.
In the process for the manufacture of alkali metal dithionites from an alkali metal salt of formic acid, an alkali metal hydroxide, carbonate or bicarbonate, and sulfur dioxide, the product precipitates in an alcohol/water solution. Upon completion of the dithionite reaction, the product is separated from the reaction filtrate, also termed the mother liquor, by filtration. The filter cake is washed with alcohol to remove the adhering filtrate, and the product is dried. The alcohol in both the filtrate and the wash alcohol is purified for re-use by distillation. The water phase from the distillation is disposed of as a co-product of the manufacturing process. As such, this material has little or no commercial value. It does contain a mixture of metal salts which include formate, metabisulfite, sulfite, thiosulfate, and sulfate. Formate, metabisulfite, and sulfite are either fed directly to the synthesis as raw materials, or are produced as intermediates in the production of dithionites.
It is well known in the manufacture of dithionites that a portion of the dithionite product decomposes during the course of the reaction to form thiosulfate. Furthermore, this decomposition is autocatalytic with respect to thiosulfate; as the concentration of thiosulfate increases, so does its rate of formation. For this reason, the aqueous co-product containing thiosulfate cannot be re-used in the process.
It is also known that certain organic compounds are capable of reacting with or complexing thiosulfates. For example, U.S. Pat. No. 4,622,216 describes a method in which certain organic compounds are added during the course of producing dithionites to react with thiosulfate and thus minimize the decomposition reaction. These thiosulfate-reactive compounds include epoxy compounds such as ethylene oxide, propylene oxide, butyl and isobutyl oxide, epichlorohydrin, and epibromohydrin as well as halogenated hydrocarbons of the general formula RX or XRX, where R is an alkyl group of carbon number 1 to 8, or an allyl, methallyl, or ethylallyl group, and X is a halogen.
When such thiosulfate-reactive compounds are added to a batch reactor, they destroy thiosulfate ions as they are being formed within the reaction vessel and minimize destruction of the sodium dithionite product. The yield of anhydrous sodium dithionite is thereby increased.
Japanese Patent Publication No. 28,397/75 teaches a process for manufacturing anhydrous sodium dithionite in an alcohol/water solvent from sodium formate, an alkali compound, and sulfur dioxide, followed by filtering the sodium dithionite crystals from the mother liquor. The publication discloses a method for recycling a portion of the reaction filtrate with reduced distillation of the filtrate by treating the filtrate with 1-to-4-fold excess on a molar basis of ethylene oxide, propylene oxide, or a mixture thereof over the amount of sodium thiosulfate contained in the reaction filtrate and by allowing the reaction mixture to stand for several hours at room temperature. The reaction filtrate is combined with the methanol used in washing the separated crystals of sodium dithionite to form a mixture of which a part is distilled to recover the methanol and isolate the additional product, which is discarded, of sodium thiosulfate and ethylene oxide or propylene oxide.
Japanese Patent Disclosure No. 110,407/83 teaches a method for producing dithionites by reacting a formic acid compound, an alkali compound, and sulfur dioxide in a water-organic solvent mixture and by adding an epoxy compound, a halogenated hydrocarbon of the general formula R--X, or a mixture of two or more compounds of these types to the reaction mixture in the final stage of the reaction. Suitable epoxy compounds include ethylene oxide, propylene oxide, butylene oxide, isobutylene oxide, styrene oxide, cyclohexene oxide, epichlorohydrin, and epibromohydrin. In the halogenated hydrocarbon, R is a primary or secondary alkyl group having 1-8 carbons, an allyl group, a 2-methylallyl group, or a 2-ethallyl group, and X is a halogen. The filtrate obtained by isolating the dithionite crystals, the organic solvent used for washing the crystals, or a mixture thereof is recycled and reused in the reaction. Both the filtrate and the washing liquid were demonstrated to be equivalent to distilled methanol as the organic solvent for producing sodium dithionite.
In European Patent Publication No. 68,248 and in U.S. Pat. No. 4,388,291, a process is disclosed for producing anhydrous dithionite in which the washing liquid discharged from the washing step is sequentially divided into two portions, a first discharge liquid and a second discharge liquid, the former being treated to convert undesirable substances inhibiting the production of dithionites into substances which do not exert an adverse influence on the production of dithionites by adding an organic compound selected from the group consisting of compounds represented by Formulas I and II and cyclohexene oxide. Formula I is as follows: ##STR1## wherein R.sub.1 group containing from 1 to 8 carbon atoms, a halogenated alkyl group containing 1 or 2 carbon atoms, a phenyl group, or a substituted phenyl group. The compound represented by this formula includes ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, and styrene oxide. Formula II is as follows: EQU R.sub.2 --X,
wherein R.sub.2 is a primary or secondary alkyl group containing from 1 to 8 carbon atoms, an allyl group or a 2-methylallyl or 2-ethallyl group, and X is a halogen atom. Suitable compounds include methyl iodide and allyl chloride.
A portion of the treated first discharge liquid was mixed with nearly twice as much of the untreated second discharge liquid and used to prepare sodium dithionite after adjusting for the amount of water in the discharge liquid mixture. The resulting purities and yields for the sodium dithionite product were substantially identical to those obtained with pure methanol.
U.S. Pat. No. 4,693,880 describes a dithionite production process in which the washings first discharged are distilled by a conventional method for recovering the organic solvent, and the subsequent washings are used as the reaction solvent without distillation.
Japanese Patent Publication No. 079,805/83 teaches the addition of an epoxy compound (selected from the group comprising epichlorohydrin, epibromohydrin, butylene oxide, propylene oxide, ethylene oxide, styrene oxide, and cyclohexene oxide) and acid (to bring the pH below 6) to the mother liquor and recycling the treated solution to the next dithionite production stage.
Japanese Patent Publication No. 091,004/83 teaches the addition of propylene oxide or ethylene oxide to the wash liquid and recycling the treated liquid to dithionite production.
Japanese Patent Publication No. 110,406/83 relates to washing the filtered dithionite crystals with an organic solvent containing an epoxy compound or a hydrocarbon halide and recycling the used washing solvent to obtain dithionite of high purity in high yields.
The objective in all of these patents is to recycle a portion of the filtrate, or the wash methanol, or both. The filtrate contains all of the unused raw materials, as much as 25% of the quantity fed to the batch reaction, plus most of the decomposition products. The filtrate also contains all of the water fed to the batch as aqueous solutions of both sodium hydroxide and sodium formate plus the water of reaction produced in the batch as shown in the overall chemical reaction: EQU NaOH+HCOONa+2SO.sub.2 .fwdarw.Na.sub.2 S.sub.2 O.sub.4 +CO.sub.2 +H.sub.2 O.
It also contains all of the alcohol fed to the batch. Since there is a very narrow range of water to alcohol ratio appropriate to the manufacture of sodium dithionite via the formate process, it is patently impossible to recycle all of the filtrate to the next batch. If attempted, no water in which to dissolve the sodium hydroxide and sodium formate would be available, nor would alcohol be available in which to dissolve the sulfur dioxide used as a raw material. Finally, if these obstacles were somehow overcome, the water made via reaction in the recycle batch would create an excessive water to alcohol ratio and severely damage or destroy the product quantity and quality.
In order to recover for re-use all of the unused raw material present in the filtrate, it is necessary to do two things:
A. remove all of the water except that quantity which would normally be present with the raw materials in the filtrate if they were virgin raw materials, and PA0 B. prevent to the greatest extent possible, the formation of sodium thiosulfate both during the batch reaction and during subsequent processing of the filtrate to produce the partially dehydrated material appropriate for re-use to make additional sodium dithionite. PA0 A. determining the contents of alkali metal formate and equivalent alkali metal hydroxide in the co-product; PA0 B. determining the quantity of water normally associated with these alkali metal compounds as used in the batch reactions for making anhydrous alkali metal dithionites; PA0 C. heating the co-product and evaporating water therefrom, (preferrably at a retention time of one minute or less) until only the determined quantity of water remains with the determined contents, to produce concentrated co-product; PA0 D. selectively admixing the following materials, at standard feed rates, under standard heating conditions, and at standard pressures, to produce a succeeding batch reaction mixture: PA0 E. heating the mixture to the standard reaction temperature and maintaining this temperature throughout the course of the reaction to produce a completed reaction mixture containing fully reacted alkali metal dithionite; PA0 F. cooling the reaction mixture; PA0 G. filtering the cooled reaction mixture to produce crude alkali metal dithionite and reaction filtrate; PA0 H. separately distilling from the reaction filtrate to produced methanol and a aqueous co-product which may be treated again as previously described or purged as a waste from the system.
Whether a thiosulfate-reactive compound is added during the dithionite-producing reaction as taught in U.S. Pat. No. 4,622,216, to the reaction filtrate before re-use thereof as taught in Japanese N. 110,407/83, or to the first wash discharge liquid before re-use thereof as taught in Japanese 091,004/83, decomposition of dithionite and formation of thiosulfate continues to occur after the compound has been consumed. If the reaction filtrate is distilled to produce co-product, such thiosulfate formation also continues to occur so that the co-product is not usable as a raw material for the reaction.
Nevertheless, the necessary raw materials to make sodium dithionite are present in the filtrate and in fact some sodium dithionite is produced during distillation. The sodium dithionite that is produced quickly decomposes to form, among other compounds, sodium thiosulfate, because sulfur dioxide that is present as sodium bisulfite in the filtrate is released during distillation. Under typical conditions, the amount of sodium thiosulfate increases about 20-25% during distillation.
It accordingly seemed reasonable that the same compounds could be used to react with thiosulfate present in the largely aqueous co-product, thereby making it possible to re-use the valuable compounds present in the material for manufacture of additional dithionite. However, all of the water that is originally present in the reaction mixture and all of the water formed by chemical reactions during such manufacture is inherently isolated in the co-product. An amount of water must consequently be removed from the co-product which represents the water made during the dithionite reactions plus that associated with the original feed solutions, leaving behind only the water associated with the equivalent amount of raw materials contained in the co-product.
A process for treating the co-product that can enable it to be usable as a raw material for the dithionite-producing reaction is accordingly needed.